1. Field of the Invention
The present invention relates to a sampling system suited for frequency-converting a modulated signal into a low frequency and a radio receiving device using the sampling system.
2. Description of Related Art
FIG. 1 shows the typical structure of a receiving device of a portable telephone and other mobile communication radio equipment. It is a radio receiving device of a double super-heterodyne system, wherein a modulated signal received by an antenna 11 is amplified by a low noise amplifier (LNA) 12 and is then input to a band-pass filter 13 having a relatively moderate characteristic for suppressing an image signal. The signal output from the band-pass filter 13 is frequency-converted in a first mixer 14 by a local signal from a local oscillator 15 into a signal having a first intermediary frequency. The signal frequency-converted by the first mixer 14 is input to a second mixer 17 via an IF filter 16 to be frequency-converted into a signal having a second intermediary frequency which is lower than the first one by a local signal from a local oscillator 18. The signal frequency-converted by the second mixer 17 is input to an A/D converter 20 via an IF filter 19 to be converted into a digital signal. The digital signal output from the A/D converter 20 is then demodulated by a demodulator 21.
Here, it is very difficult to reduce the size of the circuit because the mixers 14 and 17 are composed of analog devices in general. Although mixer circuits which are suitable for the miniaturization of the circuit have been proposed and put into the practical use so far, all of them have had a problem that the power consumption is large or the circuit is complicated because they are intended to attain an enough dynamic range and performances such as low distortion characteristics.
Further, the IF filters 16 and 19 are required to have a highly accurate frequency selectivity in order to select a signal having the frequency of the received channel and to remove the signal having the image frequency in the double super-heterodyne system as shown in FIG. 1. Because an IF filter needs to be composed of passive parts such as a ceramic filter in many cases to enhance the frequency selectivity, the size of the filter becomes large and it is not suited for the use of the radio receiving device of portable equipment which are required to be miniaturized.
Meanwhile, there is a down-sampling system as shown in FIG. 2 as another method for constructing the radio receiving device. A modulated signal received via an antenna 21 is amplified by a low noise amplifier (LNA) 22 and the unnecessary frequency component is removed by a band-pass filter 23. Then, it is down-sampled in a sampling frequency which is lower than a center frequency of the modulated signal by an A/D converter 24 to be converted into a digital signal. The digital signal converted by the A/D converter 24 is demodulated by a demodulator 25.
Although the structure of the whole system may be miniaturized because no mixer nor IF filter are necessary in the down-sampling system shown in FIG. 2, there has been a problem that the band-pass filter 23 is required to have a high frequency selectivity and the size of the filter becomes also large because the noise signal (hereinafter referred to as aliasing noise) aliased due to the sampling in the A/D converter 24 needs to be removed by the band-pass filter 23. There has been also another demerit that the noise figure (NF) becomes worse because the A/D converter 24 is required to use a high down-sampling ratio in this arrangement.
As described above, the radio receiving device of the prior art double super-heterodyne system has had the problems that (1) because the mixer is required to have the wide dynamic range and the low distortion characteristic, the mixer requires the large quantity of power consumption and complicates the circuit structure and (2) the IF filter (e.g. the ceramic filter) having the high frequency selectivity is required to select the frequency signal of the received channel and to remove the signal of the image frequency, increasing the dimension of the filter required here.
The radio receiving device of the system of down-sampling the modulated signal directly by the A/D converter has had the problems that the dimension of the filter increases and the noise figure becomes worse due to the high down-sampling ratio, i.e. the generated noise increases, because the filter having the high frequency selectivity such as the ceramic filter or a dielectric filter is required in the radio frequency signal processing section in order to remove the aliasing noise generated during the down-sampling.